In recent years, size reduction has been rapidly progressing in optical devices for digital cameras, cellular phones, and the like. Making optical devices compact requires making the lenses used in them thin, and making lenses thin requires using optical glasses with high refractive indices.
On the other hand, as a technology that allows comparatively easy molding of glass into difficult-to-form shapes, such as aspherical surfaces, much attention has been paid to so-called press-molding (precision press-molding), whereby glass heated to above its softening point is pressed to be directly molded into a lens by use of a heated press mold consisting of a pair of upper and lower pieces.
Press-molding roughly divides between a re-heating method and a direct-press method. In the re-heating method, a gob preform or polished preform having approximately the shape of the end product is fabricated, is then re-heated to above the softening point, and is then pressed to be molded into the shape of the end product by use of a heated mold consisting of upper and lower pieces. On the other hand, in the direct-press method, molten glass is directly dripped into a heated mold from a glass melting furnace and is then pressed to be molded into the shape of the end product.
In the direct-press method described above, when the molten glass is dripped, typically, a nozzle made of platinum or the like is used. The weight of the glass dripped is controlled by controlling the temperature of the nozzle. For glasses with low liquid phase temperatures (TL), the nozzle temperature can be set in a wide range of temperature, from high to low, and this makes it possible to fabricate optical elements of varying sizes, from large to small. By contrast, for glasses with high liquid phase temperatures (TL), failing to keep the nozzle temperature equal to or above the liquid phase temperature (TL) causes the glass to devitrify, and this inconveniently hampers stable dripping.
Moreover, glasses with high liquid phase temperatures (TL) themselves need to be at high temperatures when dripped, and this makes the press mold prone to oxidation at the surface and a change in the metal composition, shortening the life of the mold. This leads to higher production costs. Molding may be performed in an atmosphere of an inert gas such as nitrogen to alleviate deterioration of the mold, but then controlling the atmosphere requires complicated molding equipment, and in addition the inert gas inflicts its own running cost, leading to higher production costs. Thus, glasses used in press-molding are required to have as low a liquid phase temperature (TL) as possible.
Molding an optical glass with a high refractive index as mentioned above by press-molding involves using a glass ingredient containing PbO or TeO2. Today, however, PbO and TeO2 are suspected to be hazardous to human health, and technologies have been studied and proposed for obtaining glasses with high refractive indices without using PbO or TeO2 (see, for example, Patent Documents 1 to 4 listed below).